Tantalum clad niobium

ABSTRACT

A composite metal article is made by applying tantalum metal sheet to the surface of niobium metal and the two metals are diffusion bonded in a protective environment. The bonded metals may be cold worked to produce the final fabricated article. The final article is coated with an aluminide or a silicide coating.

United States Patent Hagan et al.

[451 Dec. 19, 1 9 7 2 [54] TANTALUM CLAD NIOBIUM [72] Inventors: MelvinA. Hagan, 6961 Crest Rd., Palos Verdes Peninsula; Roy 1. Batista, 509Avenida Mirola, Palos Verdes Estates, both of Calif.

[52] U.S.Cl ..29/19s,29/19s 51 lnt.Cl. ..B32b 15/00 [58] Fieldofsearch..29/19s,19s,194

[5 6] References Cited UNITED STATES PATENTS 2,771,666, 11/1956 Campbellet al. nings r98 3,170,234 2/1965 Tarr ..29/488 3,249,462 5/1966 Jung eta1. ...29/195 X 3,446,606 5/1969 Friedrich et al. ..29/l95 3,595,6337/1971 Friedrich et a1. ..29/198 Primary Examiner--L. Dewayne RutledgeAssistant Examiner-E. L. Weise Attorney-Daniel T. Anderson, Alan D.Akers and James V. Tura 57] ABSTRACT A composite metal article is madeby applying tantalum metal sheet to the surface of niobium metal and thetwo metals are diffusion bonded in a protective environment. The bondedmetals may be cold worked to produce the final fabricated article. Thefinal article is coated with an aluminide or a silicide coating.

2 Claims, N0 Drawings TANTALUM CLAD NIOBIUM This is acontinuation-in-part of application Ser. No.

' 809,039, filed Mar. 20, 1969 now (1.8. Pat. No.

3,579,808, dated May 25,1971. 1

As technology advances it becomes more imperative to produce materialswhich can withstand higher temperatures. Many ceramics and cermets havebeen produced for a variety of applications in recent years to push theoperating temperatures to ever higher levels.

Aside from the major consideration of thermal stability of a materialthere also exists a problem of chemical stability in an oxidizingatmosphere. To be most useful,

a material must not melt or distort at higher operating temperatures,and in addition, the material must not decompose. Thus, for example,many portions of tected by coatings.

It has now been discovered that by coating tantalum onto niobium ahigher temperature resistant coating which more fully utilizes theirinherent mechanical properties may be obtained. By diffusion bonding athick tantalum sheet onto a niobium plate and reducing the composite bycold rolling, a protective coating may be produced which prevents theoxidation and embrittlement'of niobium while increasing their upper usetemperature limits for structural applications. Cladding of niobium withtantalum in a fairly massive form, with subsequent reduction by coldrolling, is feasible because of closely similar thermal expansionproperties, excellent ductility, and similar working properties offabricability. Tantalum has a higher modulus of elasticity and a highertensile strength, which partially offsets its weight disadvantage ofbeing nearly twice as dense as niobium. Perhaps the most important:factor is that protective coatings permit higher use temperatures ontantalum than on niobium. Molten niobium oxide, which forms above about2,700 F, exerts a strong fluxing action on protective coatings and canliterally remove the coating as a droplet runs across the coatedsurface; This problem is considerably lessened with coating tantalumsince tantalum oxide melts at a much higher temperature, approximately3,400 F.

Protective coatings which are suitable for application and protection ofthe tantalum cladding are silicides and aluminides. Typical silicideprotective coatings comprise, by weight, approximately 10 percent toapproximately 30 percent chromium. and approximately 50 percent toapproximately 75 percent silicon with the balance comprising hightemperature metals selected from the group consisting of titanium, iron,vanadium, molybdenum, tungsten, hafnium, tantalum, and aluminum.Aluminides are used less frequently for high temperature applications,however, they are formed similarly to the silicides. Typically, thesecoatings diffuse into the tantalum cladding approximately 1 to 2 milsand are built up on the surface 3 to 4 mils to give a total protectivecoating thickness of 4 to 6 mils.

In the process according to this invention, tantalum sheet is hot rolledor hot pressed bonded to niobium plate in an inert atmosphere retort orfurnace. The bonded parts are then given a diffusion heat treatment atabout 2,400 F in vacuum or inert atmosphere 'to develop a pore-freemetallurgical bond. The plate thus formed can be cold rolled to obtainthe desired sheet thickness. Parts formed from this sheet have thetantalum on the hot oxidizing atmosphere side, or if desired, both sidesof the niobium can be clad with tantalum.

Upon fabrication of the final product, the protective coating is appliedby vapor deposition or molten alloy deposition. Vapor depositionincludes processes such as chemical vapor deposition, pack cementation,fluidized bed, sputtering, and similar techniques wherein the coatingmetal is transported to and deposited on the niobium or tantalum in thevapor state. Molten alloy deposition includes slurry and dip processeswherein thecoating is applied on the niobium or tantalum while in themolten state.

The slurry application process is the most practical coating forcomplexstructures. To apply the coating, metal powders are mixed in thedesired ratio and suspended in an organic vehicle reform a slurry. Theslurry is applied on the metal or alloy to be protected by spraying,dipping, or brushing. The slurry coated item is then heated to a moltenstate in an inert atmosphere thus permitting the coating to react withand diffuse into the surface to be protected, forming a higher meltingprotective complex.

The present invention will be better understood by reference to thefollowing illustrative examples.

EXAMPLE I A 1% inch square tab of niobium (0.0622 inches thick) and a 1/2 inch square tab of tantalum (0.0105 inches thick) were bonded byheating for 2 hours at 1,000 psi pressure and 2,000 F in an inertatmosphere. The resulting tab was then given a diffusion heat treatmentin vacuum for two hours at 2,400 F. The tab was cooled and then reducedby cold rolling from a thickness of about 0.072 inches to about 0.016inches without difficulty.

The tab of bonded tantalum and niobium was coated by spraying with aslurry containing 20 grams of chromium, 5 grams of titanium, and gramsof silicon. It was then thermal cycled in an inert atmosphere to permitthe coating to react and diffuse into the surface of the tab. After onehour at 2,500 F, the furnace was cooled and the silicide coated tab wasremoved.

EXAMPLE [1 Six specimens were prepared using l-foot square tantalumsheets (0.020 inches thick) and l-foot square niobium plates (0.100inches thick). The specimens were first cleaned and then placed intitanium retorts which were sealed by welding in a vacuum chamber. Theretorts were heated to 2,200 F andthen rolled to approximately 50percent reduction in thickness. After cooling and removal from theretorts, the hot roll bonded sheets were given a vacuum diffusion heattreatment at 2,600 F for 2 hours. The sheets were then cold rolledwhereby their thickness was reduced to as little as 0.010 inches withoutdifficulty as indicated in the following table:

TABLE Specimen Average Thickness No. Sheet Nb Ta Next, a slurry was madeusing 20 grams of chromium, grams of molybdenum, and 70 grams ofsilicon. The cold rolled tabs were sprayed with the slurry and thenheated to 250 F for 1 hour in an inert atmosphere. A uniform coating ofthe silicide covered both sides of the tab. Metallographic examinationof the cold rolled sheets showed uniform reduction in thickness of theniobium and tantalum.

Microhardness test on samples showed that work hardening from coldrolling operations is easily eliminated by vacuum annealing, and thesubsequent hardness values are again typical for annealed tantalum andniobium sheet. The interface diffusion zone showed microhardness valuesbetween those for tantalum sheet and niobium sheet.

The workability of the annealed tantalum clad niobium sheet wasdemonstrated by deep drawing a 1 inch diameter, 2 inch long closed endcylinder from a 0.032

inch thick, 3 inch diameter sheet (0.005 inch Ta, 0.027 inch Nb).Measurements of a section of the drawn cylinder showed very littlereduction in thickness of the environments without catastrophic failurecaused by the fluxing action of the molten oxide, it is conceivable thatthe maximum useful temperature limit for some structures can thus besignificantly increased without suffering the weight penalty involved insubstituting tantalum for niobium.

We claim:

l. A high temperature composite structure comprising a protectivesilicide coating on tantalum cladding diffusion bonded to niobium alloy.

2. A structure according to claim 1 wherein the protective silicidecoating comprises by weight 10 to 30 percent chromium, 50 to percentsilicon, and the balance selected from the group consisting of titanium,iron, vanadium, molybdenum, tungsten, hafnium, tantalum, and aluminum.

2. A structure according to claim 1 wherein the protective silicidecoating comprises by weight 10 to 30 percent chromium, 50 to 75 percentsilicon, and the balance selected from the group consisting of titanium,iron, vanadium, molybdenum, tungsten, hafnium, tantalum, and aluminum.